1. Field of the Invention
The invention relates to the preparation of the salt of 2,2'-oxydisuccinic acid by a process which produces the salt in reasonable times and temperatures while substantially avoiding phase separation or gelation of the reaction mixture. 2,2'-oxydisuccinic acid and salts thereof are effective sequestering agents and are useful as builders in detergent compositions for household, institutional and industrial use.
2. Related Art
2,2'-oxydisuccinic acid (ODS) and salts thereof are known and are known to have utility as sequestering agents and detergent builders. A disadvantage of ODS and salts thereof as detergent builders is that they may be relatively expensive to prepare.
U.S. Pat. No. 3,128,287 to Berg discloses a preparation of ODS salt by admixing maleic acid with an excess of hydroxide of calcium, barium, magnesium or strontium in the presence of water, then heating the reaction mixture from about one day to about one month at temperatures ranging from 50.degree. C. to reflux temperatures. The process yields a mixture of malic acid and ODS. Berg's Example I teaches the preparation of ODS, wherein the aqueous mixture of maleic anhydride and calcium hydroxide is reacted at reflux (100.degree. C. for 4 days. Subsequently, ODS salt is isolated from the product containing ODS and malic acid salts.
U.S. Pat. No. 3,635,830 to Lamberti et al., discloses the process for the preparation of ODS based on the process of Berg. The patent teaches separation/purification of two diastereoisomeric forms of ODS obtained by the Berg process. The patent also discloses detergent compositions comprising ODS or salts thereof as detergent builders.
U.S. Pat. No. 4,798,907 to MacBrair et al. discloses an improvement in the ODS-forming processes of Berg and Lamberti et al., wherein an alkali metal hydroxide, e.g. sodium hydroxide, is incorporated into a starting reaction mixture which also contains a divalent metal cation. To product yields of about 80% ODS, the process generally involves reacting the mixed starting materials in water for at least 12 hours at temperatures of about 20.degree.-100.degree. C. In McBrair the amount of the divalent metal cation used in less than stoichiometric.
European Patent 206,007 to Bush teaches a process for the preparation of ODS similar to the process of the MacBrair patent. Mixtures of inorganic base with water-soluble, inorganic salts of sodium, calcium or mixtures thereof may be used. a particularly preferred reactant combination constitutes maleic acid, malic acid, calcium hydroxide and sodium hydroxide. The ODS yield of about 60% is obtained in about 6 hours.
The MacBrair et al. and Bush patents rely on incorporation of alkali metal hydroxide into the reactant mixture to minimize gelation of the mixture and to increase yields of ODS. Alkali metal hydroxides indeed produce homogeneous solutions, but unlike the alkaline earth metal hydroxides, generate higher pHs and result in a gradual decomposition of ODS under the reaction conditions employed by the MacBrair et al. and Bush patents which is believed to produce fumarate. Thus, high temperatures for long periods of time must be avoided.
A workable and cost-efficient production of ODS salt must be directed towards optimizing the process conditions in such a manner that phase separation of the reaction mixture is substantially avoided while minimizing the reaction time. There have been different approaches to the problem of producing ODS at a lower cost. However, none of these approaches has been completely satisfactory.
Accordingly, it is an object of the present invention to provide a process which produces the salt of ODS.
This and other objects and advantages will appear as the description proceeds.